RECENT PROGRESS IN PHYSICS. 387 



or in words, tlie intensity of the spark is proportional to the striking 

 distance and quantity of electricity. 

 According to equation (2), 



J = P-li. 



s 



But Riess has shown that, 



w = Qii, 



s 



is the quantity of heat which is set free in the wire by discharging 

 through it a quantity of electricity q, collected od the surface s. 



Hence if the discharge stroke of an electrical bati.ery produces a spark 

 at any interruption of the circuity the intensity of the light is propor- 

 tional to the heat tvhich the same discharge produces in a piece of wire 

 forming part of the circuit. 



At the conclusion of his memoir, Masson proposes the spark gen- 

 erated under determinate conditions as the photometric unit, by 

 which it will be possible to compare the intensity of the most diverse 

 constant sources of light with a common standard. 



SECTION FIFTH. 

 ELECTRICAL ODOR. 



§ 95. Ozone and its reactions. — When we are in the neighborhood of 

 a powerful electrical machine we perceive, when the electricity issues 

 from points, or when a series of sparks are passed from the conductor, 

 a very peculiar odor, which, for sake of brevity, we will term electrical 

 odor, or osone odor. This electrical odor is very probably that which 

 is observed after a stroke of lightning ; and which, by those who do 

 not know how to characterize it properly, is termed a sulphurous smell. 

 Schonbein observed in the vicinity of a place where lightning had 

 struck a decided odor of ozone, even some time after the stroke. 



Until recently we were quite in the dark as to the nature of this 

 odor. Some physicists supposed that it was owing to a peculiar affec- 

 tion of the organs of smell, produced by electricity ; an explanation 

 which, in addition to its error, did great injury, by preventing further 

 investigation and discusf3ion. 



Others advanced the hypothesis that electrical odor was owing to 

 fine metallic particles carried off by the escaping electricity. But this 

 view also is entirely inadmissible, because the nature of the emitting 

 points does not in the least change the nature of the odor. 



